The present invention relates to a device for detecting earth's magnetism, and more particularly to a device for detecting magnetic azimuth using a flux-gate sensor.
Heretofore, an azimuth detecting device for detecting the azimuth by measuring the earth's magnetism is widely used as an azimuth information complemental means in navigation system such as for cars, portable information equipment such a portable telephone, a PDA and so on. In the azimuth detecting device, the flux-gate magnetic sensor using a toroidal core having high sensitivity is generally used, and an azimuth detecting device having high accuracy is proposed in Japanese Patent Laid Open 6-50757.
The disclosed conventional device is provided with a flux-gate magnetic sensor comprising a toroidal core having an exciting coil and an axis in the Z-direction, an X-direction detecting coil having an axis in the X-direction, and a Y-direction detecting coil having an axis in the Y-direction. The device is characterized in that a gain setting circuit is provided for setting gains which are different in accordance with ranges of azimuth. Therefore, if a detected signal has an error in direction due to the variations in the sectional area of the toroidal core, the detected signal can be corrected by the gain setting circuit. Thus, azimuth detection of high accuracy can be realized.
However, in the proposed azimuth detecting device, since there is provided the toroidal core and two detecting coils surrounding the toroidal core, it is difficult to miniaturize the device. Therefore, the device is unsuitable to be mounted in the portable information equipment such as a portable telephone, a PDA, and so on, the demand of which has increased rapidly in recent years. In addition, the mounting of the gain setting circuit causes the signal processing circuit to be complicated. Hence there are fears about increase of the equipment mounting area and the manufacturing cost.
Under such a situation, a miniaturized magnetic azimuth detecting device is proposed in Japanese Patent Laid Open 2002-243818.
In the geomagnetic azimuth detecting device, a magnetic detecting portion is formed by superimposing a plurality of substrates, interposing a toroidal core formed by an amorphous core. The block of the substrates comprises an exciting coil substrate, an X-direction magnetic detecting coil substrate, and a Y-direction magnetic detecting coil substrate.
There is provided a signal processing circuit which comprises a first analogue switch, second analogue switch, first integrated circuit for integrating the output of the first analogue switch, second integrated circuit for integrating the output of the second analogue switch, difference amplifier for amplifying the difference between the output of the first integrated circuit and the output of the second integrated circuit and an A/D converter for converting the output of the difference amplifier into a digital signal.
Since the exciting coil wound on the toroidal core and two coils of X and Y directions are formed by the superimposed substrates, the magnetic azimuth detecting device may be miniaturized, thereby enabling the mounting of the device in a portable information equipment.
However, since the device is provided with the circular toroidal core, there is a limit on downsizing of the device. On the other hand, although the toroidal core is made of an amorphous material, the amorphous material is liable to be magnetized because of large coercive force, so that the detected value is apt to have an error due to a hysteresis. Furthermore, it is necessary to provide analogue switches and the difference amplifier in the detecting circuit, which renders the circuit composition complicated and the manufacturing cost increased. In addition, driving current of several ten milliamperes is necessary for magnetic saturation of the toroidal core. Consequently, there are problems in mounting the device on the portable electronic equipment from the point of power consumption. In particular, in order to realize a headup display on the portable telephone with a GPS, it is required that the geomagnetic azimuth detecting device is further downsized and the power consumption is more decreased.